Temperature compensated cavity resonator



Aug. 15, 1961 M. w. sT. CLAIR TEMPERATURE COMPENSATED CAVITY RESONATORFiled April 24, 1958 I INVENTOR.. Maurice W St. Clair BY a) A 2,996,690TEMPERATURE COMPENSATED CAVITY RESONATOR Maurice W. St. Clair, Palo'Alto, Calif., assignor to Varian Associates, Palo Alto, Califi, acorporation of California Filed Apr. 24, 1958, Ser. No. 730,609 8Claims. (Cl. 333- 83) This invention relates in general to ultra highfrequency apparatus and more particularly to cavity resonators for use,for example, as filters or as stalos in stabilizing the frequency oflocal oscillator sources such as reflex klystrons and the like.

It is desirable that cavity resonators or stalos of the present type betunable over a range of resonant frequencies and, once tuned, that theyremain fixed in dimension during use so that their resonance frequencywill not change. However, these cavity resonators undergo a wide rangeof operating temperatures and are constantly changing in dimensions dueto expansion and contraction of their structural parts. Varioustemperature compensating schemes have been utilized in the past tomaintain the cavity resonator frequency constant over a range ofoperating temperatures. It is the main object of the present inventionto provide a cavity resonator incorporating novel, improved tuningapparatus and temperature compensation features.

One feature of the present invention is the provision of a novel cavityresonator constructed of parts made of materials having differentcoefiicients of expansion and so interrelated that at least one of themain structural walls of the cavity resonator will move in variableaccordance with the temperature changes of the cavity resonator tomaintain a fixed operating frequency; or some predetermined fixedtemperature coefiicient of frequency different from zero, if desired.

An other feature of the present invention is the provision of a novelcavity resonator construction of the above featured type wherein the oneend wall of the cavity resonator is circular and is mounted at itsperiphery on a rim made of a metal of diiferent coefficient ofexpansion, the end wall and rim being so formed and dimensioned thatradial expansion of the wall results in an axial movement of the walldue to its interaction with the rim material.

Still another feature of the present invention is the provision of acavity resonator incorporating a novel tuning plate construction forselectively changing the dimensions of the cavity resonator.

These and other features and advantages of the present invention will bemore apparent after a perusal of the following specification taken inconnection with the accompanying drawings wherein,

FIG. 1 is an elevation view of a cavity resonator which embodies thepresent invention mounted on a klystron oscillator for stabilizationpurposes,

FIG. 2 is a longitudinal cross-section view of the cavity resonatortaken along section line 2-2 of FIG. 1,

FIG. 3 is a cross-section view of the tuner mechanism taken alongsection line 3-3 in FIG. 2,

FIG. 4 is an enlarged cross-section view of a portion of one side Walltaken along section line 44 in FIG. 2

Patented Aug. 15, 1961 showing the construction utilized for temperaturecompensation,

FIG. 4a is a vector diagram illustrating the direction of movements ofthe temperature compensation plate,

FIG. 5 is a modification of the structure shown in FIG. 4, and

FIG. 6 shows the manner in which the ring 34 is assembled to the centerportion 33.

Referring now to the drawings, the cavity resonator utilized to describethe present invention comprises a hollow cylindrical main body member 11made, for example, of Invar which has a low thermal coefficient ofexpansion. A pair of coupling holes 12 and 13 which may be sealed withdielectric windows to permit hermetic sealing of the cavity, if desired,are provided in the sides of the main body cylinder. Input and outputwaveguides 14 and 15, also of Invar, are brazed to the main body overthese coupling holes. Flanges 16 and 17 of, for example, steel or brass,are brazed to the outer ends of the waveguides and serve to couple thecavity resonator to the input and output circuit members such as thereflex klystron 20. End plates 13 and 19 are mounted over the open endsof the hollow cylinder 11 by welding at junctions 21, it beingunderstood that these end plates may be screwed to body 11 or secured inany other effective manner. The end plates are provided with stiffeningribs 21. End plate 19 is of the same material as the main body 11, theconstruction of end plate 18 being described in more detail below.

Mounted in a hollow cylindrical extension 22 in the axial center of oneof the end plates 18 is a screw tuner mechanism comprising a circulartuner plate 23 welded to one end of a cylindrical flexible bellows 24,the other end of which is welded to an annular lip 25 in the extension22. The bellows 24 permits hermetic seal of the cavity resonator ifdesired. The upper portion of the bore in the cylindrical extension 22is threaded and carries a hollow cylindrical tuner screw member 26, theinner end of the tuner screw member 26 being adapted to engage the tunerplate 23. One end of a locking screw 27 is welded to the tuner plate 23and extends up within the hollow tuner screw member 26, the screw 27being provided with a lock nut 28 and washer 29 adapted to engage aledge 31 on the tuner screw member 26. The plate 23 may be movedinwardly into the cavity resonator by screwing the tuner member 26 intothe extension 22. When the plate 23 has reached its desired position,the screw 26 is locked by means of lock nut 32. The plate 23 is pulledup firmly against the inner end of the tuner screw 26 by tightening locknut 28 down on screw 27. Thus the position of the plate 23 within thecavity resonafor is determined by the positioning of the tuner screw 26and the locking screw 27 is utilized to lock the plate firmly againstthe tuner screw 26. The lip 25 serves to limit the distance that tunerscrew 26 can be moved into the cavity resonator to therefore preventoverstretching the bellows 24.

The same end plate 18 is constructed in a novel manner to providetemperature compensation for this cavity resonator. The large centralportion 33 of this end plate (including the above described tunermechanism) is made of steel or other high expansion material and isbrazed to an outer rim or ring portion 34 which is of a material such asInvar having a very low thermal coefiioient of expansion relative to thematerial of the central portion 33. The central portion 33 is providedwith a peripheral section 35 joined to the central or main section 33 ata relatively thin annular junction point 36 (FIG. 4). It is noted thatthe tuner structure shown in the plate 18 in FIG. 2 has been omitted inFIGS. 4 and 5 for the purpose of giving a clearer picture of thetemperature compensation feature. The annular Invar ring 34 also has aninner circumferal section 37 which is joined to the main section of theInvar ring at a relatively thin junction point 38. The upper surface 40of the peripheral section 35 is directed at an angle relative to theaxis line 39 and mates with the end surface of the circumferal section37 which extends at the same angle, these two surfaces being securelybrazed together. The reason for this angular mating surface 40 will bedescribed in more detail below.

The Invar ring 34 and main body 11 to which the ring 34 is secured havea relatively slight expansion over the temperature range of operation ofthe cavity resonator due to the low coefficient of expansion of Invar.Therefore the junction point 38 remains substantially stationary in theradial direction and in the axial direction relative to the axis 39during all temperatures of operation. The steel central portion 33,however, has a relatively high thermal coeflicient of expansion and itexpands radially from the central axis 39 a relatively large distance aswell as inwardly into the cavity resonator. Thus the junction point 36moves radially outward from the axis 39 and axially inward into thecavity resonator with temperature expansion and vice versa withtemperature contraction. The flexibility at points 36 and 38 allowsconsiderable bending and flexing of the Invar ring 34 and steel center33 during the relative expansion and contraction between the two. Thejunction point 36 of the center portion 33 therefore swings arcuatelyabout the junction point 38 in the inner ring 34 as indicated by thevectors 41 and 41' (see FIG. 4a). It is noted that the motion indicatedby the vector 41' has a downward vertical component 42 when the centerportion 33 is expanding and vector 41 has an upward vertical component43 during contraction. Thus the inner surface 44 of the steel centerportion 33 tends to move in a direction inwardly of the cavity resonatoras center portion 33 expands. This inward motion results from thecomponent 42 increased by the expansion of the metal in the spacebetween surface 44 and the mating surface 40. Conversely, duringcontraction the center portion 33 moves outwardly of the cavityresonator.

The amplitude of the vertical components 42 and 43 of the movementvector between junction point 38 and junction point 36 may be adjustedby changing the angle between the line drawn through the points 36 and38 and the horizontal and thus the ratio of distance of movement todegree of temperature change may be varied. Since such motion changesthe resonant frequency of the cavity the resulting eifect is to produceany desired rate of change of cavity frequency with temperature; and, ifdesired, to exactly compensate for all other temperature dependentfrequency changes in the cavity, such as the frequency Changes due tothe small residual coeflicient of the Invar body material. Changes inangle 6 may be made by properly dimensioning ring 34 and center portion33 including their extensions 35 and 37 before assembly. The angle 0 mayalso be changed or adjusted by a simple expedient after completeassembly of the cavity resonator if a change or correction intemperature compensation is desired. This variation may be accomplishedby making a cut 45 in the plate 33 near the junction 36 as shown in FIG.5. This cut shifts the most flexible point 36 of the center portion 33to the left relative to the position shown in FIG. 4 and thus results inan increase in angle 0.

It is noted that the configuration in FIG. in which 9 has been madelarger than 90 results in an outwardly directed vertical component ofmovement during temperature expansion and an inwardly directed componentduring contraction. This is opposite to the movements of theconfiguration of FIG. 4.

Also, the point 38 may be shifted relative to the point 36 by shavingthe surface 46 (see FIG. 5) and thus decreasing the angle 0.

The slant in the surface 40 of section 35 and in the mating surface ofsection 37 aids in assembling the central portion 33 to the ring 34. Toassemble, the ring 34 is placed on the surface 40 and a weight 47 placedon top of the ring to urge it into close contact with this surface (seeFIG. 6). A ring of silver solder 48 is placed at the V junction of thesection 35 and the section 37 and the assembly placed in a brazingfurnace. As the materials heat up the central portion 33 and section 35radially expand and the ring 34 having a low coeflicient of expansion,slides down on the surface of section 35. The silver solder flowsbetween the mating surfaces. On cooling of the assembly, the soldersolidifies and, although the central portion 33 contracts, friction andthe downward pressure on ring 34 prevent the section 37 from sliding upthe surface of section 35 and weakening or breaking the brazed joint.

Since many changes could be made in the above construction and manyapparently widely dilferent embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A temperature compensated high frequency cavity resonator comprisinga main body forming the cavity resonator and a movable wall in said mainbody for temperature compensation of said cavity resonator, said wallincluding an outer annular rim portion made of material of one thermalcoeflicient of expansion, said rim portion comprising a main sectionsecured to said main body and an inner circumferal section with anannular junction between the sections, said wall also including acircular central portion made of a material of a different thermalcoefficient of expansion comprising a main central section and aperipheral section with an annular junction therebetween, the peripheralsection of said central portion being securely aflixed to the innercircumferal section of said rim portion, relative movement between saidrim portion and said central portion during the expansion andcontraction of said cavity resonator during temperature changesproducing a flexing of said portions at said junctions with a resultantmovement of the wall section within the cavity resonator.

2. A temperature compensated high frequency cavity resonator as claimedin claim 1 wherein the mating surfaces at which said peripheral sectionand said inner circumferal section are joined are directed at an obtuseangle relative to the plane of said wall.

3. A temperature compensated high frequency cavity resonator comprisinga main body forming the cavity resonator and a wall in said main bodyadapted for movement in the body for temperature compensation of saidcavity resonator, said wall comprising a rim section affixed to saidmain body made of material having one thermal coefficient of expansionand a central section securely aflixed to said rim section and made of amaterial having a different thermal coefiicient of expansion, said wallhaving a relatively flexible portion at the junction of said rim sectionand central section, the flexible portion bending during radialexpansion and contraction of said central section during temperaturechanges and being arranged to move the central section of said wallwithin said cavity resonator to effect temperature compensation of thecavity resonator.

4. A temperature compensated high frequency cavity resonator as claimedin claim 3 wherein said central section of the wall is positioned moreinwardly of the main body than said rim section such that said flexibleportion extends in a direction which forms a substantial angle relativeto the plane surface of said wall.

5. A cavity resonator apparatus as claimed in claim 1 wherein said rimportion and said main body are made of material having substantially thesame thermal coefiicient of expansion.

6. A cavity resonator apparatus as claimed in claim 5 wherein said rimand main body material is Invar.

7. A cavity resonator apparatus as claimed in claim 3 wherein said rimsection and said main body are made of material having substantially thesame thermal coefiicient of expansion.

8. A cavity resonator apparatus as claimed in claim 7 wherein said rimand main body material is Invar.

References Cited in the file of this patent UNITED STATES PATENTS2,486,129 De Walt Oct. 25, 1949 2,495,744 Litton Ian. 31, 1950 2,507,426Turney May 9, 1950 2,752,576 Hilliard June 26, 1956 2,880,357 Snow Mar.31, 1959

